Interpretive Summary: Biological control, the utilization of natural enemies to reduce the damage caused by noxious organisms to tolerable levels, is an important component of American agriculture. Some of the most effective biological control agents used to control insect pests of crops belong to the insect order Hymenoptera, which contains the ants, bees, and wasps. Most of these are parasitic wasps which have a conditional means of sex determination whereby fertilized (diploid) eggs give rise to female progeny and unfertilized (haploid) eggs, male progeny. In the laboratory, many species exhibit male-biased sex ratios, which hinder biological control efforts using parasitic Hymenoptera, because (1) by reducing the number of female founders and population growth rates, they can decrease the likelihood of establishment of imported species, and (2) by making the production of females prohibitively expensive, they can hinder the commercial development of species used in augmentation. A potential cause is a phenomenon known as single-locus complementary sex determination (sl-CSD), whereby sex is determined by multiple alleles at a single locus. Fertilized eggs heterozygous at the sex locus develop as females while those that are homozygous develop as diploid males, depressing the proportion of females among progeny. In laboratory studies, a team of scientists from the University of Minnesota, South Dakota State University, Kyushu University (Fukuoka, Japan) and the ARS Beneficial Insects Introduction Research Laboratory (Newark, Delaware) ruled out single-locus CSD as a sex-determination mechanism on a parasitic wasp in the family Braconidae that attacks seed weevils. This was a useful finding because it indicated other factors might contribute to male-biased sex ratios in that family of wasps, which contains many potential biological control agents.

Technical Abstract:
In the haplodiploid Hymenoptera, haploid males arise from unfertilized eggs, receiving a single set of maternal chromosomes while diploid females arise from fertilized eggs and receive both maternal and paternal chromosomes. Under single-locus complementary sex determination (sl-CSD), sex is determined by multiple alleles at a single locus. Sex locus heterozygotes develop as females while hemizygous and homozygous eggs develop as haploid and diploid males, respectively. Diploid males, which are inviable or sterile, are therefore produced in high frequency under inbreeding. CSD is considered to be the ancestral form of sex determination within the Hymenoptera because the most basal taxa have CSD while more derived groups have other mechanisms of sex determination that produce the haplo-diploid pattern without penalizing inbreeding. In this study, we investigated sex determination in Heterospilus prosopidis Viereck, a parasitoid from a relatively primitive subfamily of the Braconidae, a hymenopteran family having species with and without CSD. By comparing sex ratio and mortality patterns produced by inbred and outbred females, we were able to rule single-locus CSD out as a sex determination mechanism in this species. The absence of sl-CSD in H. prosopidis was unexpected given its basal phylogenetic position in the Braconidae. This and other recent studies suggest that sex determination systems in the Hymenoptera may be evolutionary labile.